Studies will be performed of the effect of a factor isolated from plasma of dogs and rats on vascular reactivity in the hind limb of the rat. Evidence has accumulated to suggest that hypertension may be promoted by a circulating factor that inhibits vascular Na K-ATPase. We have previously demonstrated that plasma of salt loaded dogs contains a factor which cross reacts with anti-digoxin antibodies and inhibits Na K-ATPase. Extracts containing this factor have been shown to increased reactivity of third order arterioles of the rat cremaster muscle to the vasoconstrictor effects of norepinephrine, arginine vasopressin and angiotensin II. These studies will be extended by investigating the effects of these extracts on vascular reactivity of an isolated hind limb preparation in the rat. In the preparation to be used, blood is removed via the carotid artery, passed through an extracorporeal circuit, and pumped back into the iliac artery and the isolated hind limb. Vascular reactivity is assessed by measuring the changes in hind limb perfusion pressure at constant hind limb blood flow in response to direct intrailiac injection of vasoactive agents. The preparation responds to norepinephrine over a dose range of 10-200 ng and is stable in excess of six hours.